Space-saving storage of flexible sheets

ABSTRACT

A hub and deployable annular sheet folded and wrapped thereon, the folds including major fold lines lying in straight lines which are tangent to an imaginary cylinder located just inside of the hub, and the sheet material being additionally folded or pleated between the major fold lines in such a way that at each fold line only one thickness of the material is folded upon itself, and the sheet being deployable to extended position, and in some modifications held therein, by rib means lying along the major fold lines.

United States Patent [191 Scheel Nov. 19, 1974 1 SPACE-SAVING STORAGE OFFLEXIBLE SHEETS Inventor: Henning W. Scheel,

Ernst-August-Str. l8, 2 Hamburg 52, Germany Filed: Aug. 28, 1972 Appl.No.: 283,961

T BYeYgHXpiFiEaEOHfiiol-ityiati Nov. 17, 1972 Germany 2144034 B65h 45/02[58] Field Of Search 242/55, '54 R, 67.1; 135/1 R,1A,1B,1C,1D,5R,8

[56] References Cited UNITED STATES PATENTS 2,942,794 6/1960 HUSO 242/553,010,372 11/1961 Lanford 242/55 X Primary Examiner-George F. MautzAttorney, Agent, or FirmArthur E. Dowell, Ill

[57] ABSTRACT A hub and deployable annular sheet folded and wrappedthereon, the folds including major fold lines lying in straight lineswhich are tangent to an imaginary cylinder located just inside of thehub, and the sheet material being additionally folded or pleated betweenthe major fold lines in such a way that at each fold line only onethickness of the material is folded upon itself, and the sheet beingdeployable to extended position, and in some modifications held therein,by rib means lying along the major fold lines.

10 Claims, 12 Drawing Figures SPACE-SAVING STORAGE OF FLEXIBLE SHEETSThis invention relates to apparatus for storing and deploying a thinflexible sheet, particularly an annular or circular sheet of plastic orfabric material, by folding the sheet in a way resembling pleating whilewrapping it against the outer surface of a cylinder or prism to whichthe sheet is fixed in the vicinity of its center. Storage according tothe present invention is useful for many purposes, for instance, to wrapand store tents, umbrellas, sails, sheet filter materials, reflectingradiators, etc., and this apparatus is especially useful for storing anddeploying large arrays comprising solar cells mounted on a supportingsheet for use in connection with spacecraft power generation. Anotheruse is to fold and store, and to automatically deploy heat shields,micrometeorite shields, or the like.

It is a principal object of the invention to provide structural meansfor folding and wrapping circular sheets and for easily unwrapping anddeploying, preferably automatically, these same sheets without damagethereto or tangling, and preferably to provide means by which suchsheets can easily be stored again by folding and wrapping in theoriginal configuration. An essential characteristic of this type ofstorage is that the finally stored sheets must occupy only a very smallspatial volume, and that when folded and wrapped, they must be protectedfrom damage, especially environmental damage such as pressure, shock,vibration, or exposure to humidity or to other detrimental contact.

It is well known in the prior art to fold circular sheets together or toroll them up on a drum. Folding them together in multiple layers tendsto result in rather disorderly storage in which damage to the sheets islikely especially where multiple thicknesses are first piled up and thenfolded at a bend line. On the other hand, rolling unfolded sheets on adrum results in a final stored configuration which is excessivelylengthy axially of the drum.

The present invention seeks to teach an optimum way to fold and wrap asheet of material having a memory for its bend lines against the surfaceof a cylindrical hub which is fixed concentrically to the center of thesheet and has its axis normal to the surface of the sheet when deployed,regardless of which of the above practical applications the presentinvention is applied to. This is quite difficult to accomplish becauseplane sheets are only foldable along straight lines. As a result, thespiral type of wrapping as shown, for example, in FIG. 3 of US. Pat. No.3,109,608, starting precisely at the center of the sheet is out of thequestion.

The problem sought to be solved in the above US. patent using radiallyextending fold lines, can however be solved using major fold lines whichextend in a direction which is almost tangent to the hub. The presentinvention uses such major fold lines, which extend from the cylinder orhub and are disposed at a certain angle alpha with respect to radiallines extending from the center of the hub. The words almost tangent areused because the major fold lines are actually tangent to an imaginarycylinder which is somewhat smaller in diameter than the cylindricalsheet-wrapping hub itself, the imaginary cylinder being locatedconcentrically therewithin. Between these major fold lines there areintermediate fold lines which bisect the angle between two adjacentmajor fold lines and extend out to the perimeter of the circular sheet,folding the sheet material in alternate directions so as to produce apleating effect. According to the present invention there may bemultiple additional minor fold lines about which the sheet is pleated;the more such fold lines, the shorter the wrap of the sheet on the outersurface of the cylinder as measured axially of the cylinder. Whenpleated in this manner, the sheet material is then wrapped tightlyaround the surface of the cylindrical hub and the pleated surfaces arethereby laid on top of each other for support adjacent to the surface ofthe cylinder.

In order to reduce the axial length of the cylinder required to receiveand support the circular sheet when folded into pleats, the number oftimes that the sheet material is pleated along minor fold lines can beincreased, so long as folded sectors are all essentially the same widthas measured parallel to the axis of the cylinder when the pleats arewound upon it. In the several illustrated embodiments, it will benoticed that there are different numbers of pleats appearing betweenadjacent major fold lines.

Another object of the present invention is to provide a system forfolding and wrapping a circular sheet about a cylinder while at the sametime placing padding spacers between the adjacent pleat segments of thecircular sheet when it is initially folded and as it is being wrappedupon the cylinder. This is especially useful, for instance, where thepresent teaching is employed to provide a solar cell array so as to padthe individual solar cells and prevent breakage thereof during wrappingof the supporting circular sheet material onto the cylinder. When oncewrapped thereon, it is also useful to securely hold the array close tothe cylindrical hub using releasable bindings and/or plastic bandcovers, so that the stored array cannot be prematurely displaced as aresult of shocks, vibrations or accelerations.

Still another important object of the invention is to provide stiffeningmeans to supplement the memory of the pleated sheet, such as spring-typeribs or even inflatable rib tubes which can be pressurized from withinto cause them to extend outwardly of the array, these stiffening meansbeing preferably attached to the sheet material at the major folds andbeing wrapped together with the sheet material around the storagecylinder. Deployment of the sheet material can be easily permitted oreffected, by releasing the bindings and/or plastic covers and using thespring tendency or inflation of the stiffening ribs to cause them toassume straight-line configurations so as to unwrap the sheet and deployit in an approximately radial plane, thus resulting in de ployment to astable configuration which can then be used as a tent, umbrella, sail,filter, radiator, etc. This same type of circular planar configurationis also especially useful as a solar array in which solar cells areattached in a pattern to the sheet material, or in which the sheetmaterial is deployed for use as a radiation or micrometeorite shield.

It is another object of this invention to provide a structure of thecharacter set forth wherein deployment of the sheet can be retarded anddamped at a controlled rate using string-like filamentary means attachedbetween the cylinder and the sheet or the stiffening ribs to preventexcessively rapid deployment, especially where the ribs have highspring-like qualities.

Active control of the rate of deployment can be achieved for example, byusing strings which are fastened to the outer tips of the ribs and whichcan be spooled on or off of the reel means located at or inside of thehub, using brakes or motors. Refolding and wrapping of the sheetmaterial, once deployed, can be achieved remotely by using the samemotor to pull the strings back in towards the center of the hub andthereby wrap the sheet back into the stored position.

In other possible configurations of the present invention, a force fieldacting radially from the center of the structure can be used toaccomplish deployment. For instance, in a spin stabilized vehicle thecentrifugal force created by rotation of the hub and sheet may be usedto deploy the sheet and maintain it in an extended condition with orwithout stiffener ribs, or in altemative configurations it may bepossible to use electrostatic repulsion from an electrostaticallycharged hub to accomplish deployment of the sheet.

It is also satisfactory to use inflatable stiffener means built into thesheet or attached to it in such a way as to automatically stiffen it andurge it into deployment, for instance, using elongated flexible tubeswhich can be pressurized from within, perhaps using internally generatedgas or foam material so that once deployed it will remain deployed.

Moreover, the present structure and technique need not be limited to thestoring and deployment of essentially planar sheets because, forexample, parabolic sheet material structures can be stowed ina similarmanner if the over-all sheet material is provided with small elasticintermediate segments of suitable area which would be attached to themajor and to the intermediate or secondary fold lines. If this weredone, the deployment processes and structures described above can beused to accomplish non-planar resulting structures, such as parabolicdish structures capable of use as high gain RF antenna reflectors.

The present technique and structures have certain advantages which existwhether the structure is used as a tent, umbrella, sail, filter,reflector, etc. One very important advantage is that all sheet materialfolds are accomplished using only single folds where only singlethicknesses of the material on opposite sides of the fold lines andcontiguous thereto will lie against each other, as distinguished fromseveral layers of material being first stacked together and then allfolded about'a common bend, which latter type of folding tends to undulystress and stretch the outer layers of the stacked material. Since allof the pleats are single pleats, tangling during deployment is avoided,and in fact almost impossible. Another advantage of this type ofstructure is that whatever stiffening ribs are used, whether springmetal or pressurized tubes, etc., these can also be wrapped around thehub. When released, the stiffeners materially aid in the deployment ofthe structure into extended position with exceptional ease and rapidity.

Unlike some of the other storage and deployment means, especially thoseused in solar arrays, the present sheet, being supported at its center,can be stiffened by ribs which are relatively short in length so thatwhen deployed, the sheet becomes exceptionally stable. Moreover, thestiffening means can be selected and shaped in cross-section so as toprovide stiffening of the deployed sheet in any predetermined directionwhereby the sheet is held substantially planar when deployed. The sheethas a slight fish-bone pleating pattern when deployed, the pattern beingattributable to the lines about which it was folded, and these linesgive the sheet when deployed an attractive and interesting appearance.

The present invention has special advantages when applied to lightweightsolar generator arrays for space vehicles, these advantages beingespecially apparent when compared with the usual roll-up array wheresingle solar cells are mounted on large rectangular flexible sheets ofplastic, usually called blankets. In order to achieve a certain over-allarea for the array, the roll-up blanket can generally extend from thedrum on which it is rolled only to a distance which is about five timesits width, because beyond that length the blanket would have too low adegree of torsional stiffness. When the present novel structure is usedto provide a solar array of circular shape, such an array has thefollowing advantages over the conventional roll-up strip blanket array.

The outer dimensions of the circular array according to the presentinvention can be designed quite freely and without serious concern overthe axial length of the cylindrical hub on which it is to be storedafter folding, whereas a non-folded roll-up blanket array would requirea long hub whose length would be determined by the diameter of theunrolled blanket. Where space is important, as in a space vehicle, theaxial length of the drum on which the array is stored can be veryimportant.

The present storage structure provides much less friction duringdeployment between the various solar cell components and/or the sheet onwhich they are mounted, since the pleats open about the bend lines whichact as hinges and do not tend to scrape across the adjacent pleat. Thesuccessful deployment of a circular array is more reliable, and oncedeployed in the array can be maintained deployed very reliably simply bycentrifugal force in a spin stabilized unit.

In order to achieve a certain cell area of the solar array, much shorterstiffening means can be used in the present structure than in the caseof the rolled-up blanket array, thus, the bending stresses on thestiffening means can be less and they can be designed from lighterweightmaterials. The power-to-weight ratio of the solar cell can accordinglybe increased, and for the same reason the vibrational bending frequencyof the present circular solar array will be much higher as compared tothat of a roll-up array. Hence, the array will provide less interferencewith the attitude control system of a spacecraft upon which it ismounted.

Displacements of the center of gravity of a circular array according tothe present invention from its geometric center, such as might occur asa result of incomplete deployment, thermal dilatations, etc., areminimized because the array is symmetrical about its center, and ittherefore results that torques caused by solar pressures arenon-existent.

The deployed array is subject to balanced forces which extend, not inonly a few directions, but in all directions from the center, whereby itis maintained exceptionally planar.

For power transfer, no slip rings or long braided wires are necessary.

The pleats of the supporting sheet material between the folds are allstrips of the same width, and thus all of the solar cell arrays can bemounted thereto using the same jigs. Such constant-width strips can befabricated at high production rates using individual seriesconnectedthin film solar cells, which would provide for economic production of anarray.

Finally, the cylindrical shape of the stowed array makes itsincorporation as a part of a spacecraft especially easy since the stowedarray will lie neatly beneath a cylindrical shroud or fairing of thelaunch rocket. Moreover, where it is necessary to deploy a number ofsimilar solar arrays in orbit from a single spacecraft, a number ofstorage hubs, each carrying its own solar array wrapped according to thepresent disclosure, can be folded outwardly on lightweight structuralarms from the spacecraft, and then each of theflindividual arraysdeployed from this extended position.

Other objects and advantages of the present invention will becomeapparent during the following discus sion of the drawings, wherein:

FIG. 1 is a side view of a storage cylinder or hub upon which a circularsheet has been folded and wrapped;

FIG. 2 is a view similar to FIG. 1 but showing the sheet partiallyunwrapped from the cylinder;

FIG. 3 is an elevation view similar to FIGS. 1 and 2 but showing thecircular sheet fully unwrapped and deployed in a plane normal to theaxis A of the cylinder;

FIG. 4 is a partial view looking up along the axis of the storagecylinder and showing the circular sheet fully deployed;

FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 1;

FIG. 6 is a schematic partial sectional view taken along line 6-6 ofFIG. 5 but showing the sheet somewhat expanded from its normal tightlywound stored position so as to make its folds visible;

FIGS. 7, 8, and 9 are related perspective views showing a somewhatmodified embodiment of the invention in three different degrees ofdeployment, FIG. 7 showing the sheet in an early stage of deployment,FIG. 8 showing the sheet in an advanced state of deployment and FIG. 9showing sheet fully deployed;

FIG. 10 shows a still further modified version of the inventionincluding means for controlling the rate of deployment and forrewrapping the sheet around the cylinder.

FIG. 11 shows an enlarged cross-sectional view through a spring metalrib; and

FIG. 12 shows an enlarged cross-sectional view through an inflatableplastic rib.

Referring now to the drawings, FIGS. 1, 2 and 3 show a cylindrical hub 1having an outer surface against which the sheet material 2 is stowed byfolding and then wrapping it in the manner described. FIG. 5 illustratesthe fact that when the sheet is fully stowed, the outer diameter of thecylinder is only slightly enlarged because the sheet material lies flatagainst it. Thus, in stowed condition the sheet requires only a verylittle space. The structure also includes a plurality of flexible ribmembers or stiffeners 4 each of which is attached to the storagecylinder 1 at its inner end and extends therefrom outwardly along amajor fold 3a, 3b, 3c, of the sheet material 2. The sheet material isattached all along the length of its contact with each of the ribs 4.FIG. 6 is a view of the wrapped-up sheet material which has beenradially expanded to show how the sheet 2 is draped over the respectiveribs 4 at major folds 3a, 3b, 3c, Between the major folds 3, there arelocated a number of intermediate and minor folds 5a, 5b, 5c, and 5d, andthese four folds repeat between each of the major folds all the wayaround the pattern. Note that the folds 5a, 5b, and fold (down in FIGS.2 and 3) toward the viewer in FIG. 4, whereas the folds 3 and 5d foldaway from the viewer (up in FIGS. 2 and 3). There are intennediate folds5a, and 5d and there are minor folds 5b and 50 which are located so thatthey intersect the intermediate folds. Where solar cells or otherfragile components are used the individual folds of the sheet may beinterspersed with padding spacers P as shown in FIG. 6. Around the outerperiphery of the assembly there may be located a plastic retaining band7, FIG. 6, which can be held together at its ends in a suitable mannerdesignated to tightly confine the folded and wrapped circular sheet toprevent damage thereto, and in the case of a space vehicle to hold thewrapped sheet and ribs tightly against all possible displacements.

FIG. 2 shows the circular sheet partially deployed. Note that the axialheight of the wrapping on the hub cylinder 1 is labelled H and that thisheight is the same as the widths H of the strips between the fold linesas shown in FIG. 4. As shown in FIG. 11, the crosssectional shape ofeach rib 4 is arcuate so that it can be bent for wrapping around thecylinder, but so that when it is extended in a straight line it willhave stiffness as required to hold the circular sheet material firmlydeployed in a planar manner.

FIG. 3 shows the circular sheet 2 in elevation view completely deployedand supported by the arcuate stiffener ribs 4 which are mounted in thecylindrical hub 1.

FIG. 4 shows a view of the deployed circular sheet 2 as seen from abovealong the axis A of the cylinder 1, FIG. 3, thereby illustrating theextended ribs 4 as straight lines and attached to the circular sheet 2at the major folds 3a, 3b, 3c, When the circular sheet is deployed, itlies substantially transversely at the upper end 1a of the circular hubjust above the ribs 4.

The sub-folds 6a, 6b, 6c, and 6d correspond with the folds 5a, 5b, 5c,and 5d but are located between another pair of ribs 4. The intermediatefolds 5a and 6a bisect the angles 8 fonned between each pair of adjacentmajor folds 3 and are pleated in alternating directions with respectthereto. A group of solar cells 9 are shown on the fabric between theribs 4 and the minor fold 50 to show typical placement thereof.

Although at first glance it may appear that the major folds 3a, 3b, 3c,and the ribs 4 attached thereat extend in directions which are tangentto the outer surface of the hub cylinder 1, this is not true. Actually,they are tangent to an imaginary circle C which is of slightly smallerradius r than the radius R of the outer surface of the hub 1, FIG. 4.The radius r of the imaginary circle C is selected with respect to theradius R of the hub 1 such that extensions of any two adjacent majorbend lines 3 will intersect at a common point K which lies slightlyoutside the surface of the hub 1. Moreover, the ribs 4 and the majorfolds 3a, 3b, 3c, lie at a certain angle alpha with respect to a radialline L passing through the center axis A of the cylinder 1 and the pointof intersection of the ribs 4 and major fold lines 3 with the surface ofthe cylinder 1. This angle varies, depending on how many major folds andribs are employed around the periphery of the cylinder 1. The anglealpha must be carefully selected in order to provide proper geometry toachieve correct storage and deployment of the assembly. For example, iffour major folds are employed, the angle alpha should be about 52 (R/requals 1.28). If six major folds are used, as shown in FIG. 4, then theangle is about 66 degrees, (R/r equals 1.10). For eight major folds, anangle of about 71 is proper (R/r equals 1.05).

FIGS. 7, 8 and 9 show a modification of the structure illustrated inFIG. 4, in which the number of pleats is increased between each pair ofribs 14 for the purpose of shortening the height H of the sheet 12 whenit is wrapped upon the cylindrical hub 10. Initially when the sheet 12and ribs 14 are fully wrapped upon the hub 10 the length of the hub issubstantially covered in the same way as the hub 1 is covered by thesheet 2 in FIG. 1. Then, as the sheet material unwraps and becomesdeployed, the diameter of the sheet 12 becomes larger and larger whilethe covered height H" of the cylinder, FIG. 8, remains constant untilall subfolds 5b, 5c, and 5d are deployed, ie the deployment reaches thepoint K. Then H" rapidly increases as can be seen by comparing thedimension H" in FIG. 8 with the dimension H in FIG. 9 in which the sheetmaterial is fully deployed. Since the number of pleats has beenincreased as shown in FIG. 9, as compared with the number of pleatsshown in FIG. 4, the dimension H is accordingly smaller than thedimension H in FIG. 4 assuming the same outside diameter of the deployedsheet material.

Finally, FIG. 10 illustrates a still further modification of thestructure in which two things are changed. In the first place, thenumber of pleats has been still further increased to illustrate adifferent embodiment which would wrap on an even shorter length H ofstorage hub. The second difference is that in FIG. 10 means has beenadded by which a controlled rate of deployment of the sheet material canbe achieved, and by which the sheet material can be mechanicallyrewrapped into folded condition. In this illustration, the hub isgenerally referred to by the reference character 20, and the sheet isreferred to by the reference character 22. The ribs are referred to bythe character 24, and at the end of each of these ribs 24 there isattached a string 26 which extends from the tip of the rib in toward thecentral hub. Within the cylindrical hub there is a drive means, in theillustrated case a motor M capable of rotating a wind-up reel 28attached to it and lying concentric with the cylinder 20 on which thefabric wraps. Each of the strings 26 is wrapped onto the reel 28 andsecured thereto, and it will therefore be seen that starting with thestrings wound tightly on the reel and the sheet 22 folded and wrappedtightly on the storage cylinder 20, as the motor M is rotated clockwisethe strings will be paid out from the reel, thereby allowing the ribs 24to extend to their fully deployed positions at a controlled rate. On theother hand, if it is desired to return the sheet 22 to the stowedcondition, the motor can be reversed to drive the reel 28counterclockwise, thereby pulling the strings tightly in toward thecylinder and returning the sheet material to wrapped condition. Sincethe sheet material is permanently creased at each of the major and minorfolds and bends, it rolls up to its stored condition again quitereadily. In the working embodiment of the present invention, the sheetmaterial is a plastic fabric having sufficient memory to regain itsfolded position about the original creases when the ribs 24 to which itis attached are wound again onto the hub 20.

Other modifications of the present invention are of course possible, forinstance, including a modification in which the ribs, instead of beingC-shaped as shown in FIG. 11, are hollow plastic tubes 4a as shown inFIG. 12, which are inflated from within by gas pressure or by a foamsubstance to a sufficient pressure that the ribs 4a assume and retainlinear deployment. In addition, it is also possible to deploy the sheetby a force field operating substantially radially from the axis A of thecylinder, for instance, an electrostatic force or the centrifugal forcecaused by rotation of the entire assembly.

The invention is not to be limited to the exact illustrativeembodiments, for obviously changes may be made therein within the scopeof the following claims.

I claim:

1. Apparatus for storing on a hub an annular sheet in folded conditionand for deploying it into extended position, comprising:

a. a hub of diameter which is small as compared with the diameter of thesheet when extended and the hub having an axis;

b. a sheet made of thin flexible material fixed in the vicinity of itscenter to the hub such that when the sheet is in extended position itlies substantially normal to said axis of the hub;

. the sheet having multiple predetermined sharplydefined bend linesabout which the sheet is pleated by folding the sheet in one axialdirection at multiple major bend lines uniformly spaced about the sheetand then folding the sheet oppositely at intermediate bend linesbisecting the angle between adjacent major bend lines, the folds beingmade in such a way that at each bend line only the sheet materialcontiguous thereto is folded upon itself, said major bend lines eachextending from the hub as a substantially straight line which is tangentto an imaginary coaxial cylinder located within the hub and of slightlysmaller diameter and selected such that extensions of adjacent majorbend lines would intersect at common points lying near the surface ofthe hub; and

d. the pleated sheet having sufficient memory of said I sharply definedbend lines to assume said folded condition when rolled onto said hub.

2. Apparatus as set forth in claim 1, wherein the sheet is furtherpleated to form said segments by folding it in alternating axialdirections about minor bend lines which are also substantially straightlines so located on the sheet when in extended position that each minorbend line is parallel to an adjacent major bend line and that thespacings between each minor bend line and an adjacent parallel major orminor bend line is substantially a constant distance measured anywhereon the sheet.

3. Apparatus as set forth in claim 1, wherein padding spacer means areplaced between the pleated segments of the sheet when folded forstorage.

4. In apparatus as set forth in claim 1, including releasable retainingmeans surrounding the pleated and rolled sheet when in folded condition.

- 5. In apparatus as set forth in claim 1, said sheet being deployableby a radially outwardly acting force field.

6. In apparatusas set forth in claim 1, flexible tube means comprisingstiffening rib means fixed to the sheet along each major bend line andeach rib means being attached to the hub where the bend line intersectsit, the tube means being inflatable to stiffen them and deploy the sheetinto extended position.

7. Apparatus as set forth in claim 1, including springlike stiffeningrib means fixed to the sheet along each major bend line and each ribmeans being attached to the hub in a position such that when the sheetis in extended position, the rib means lies tangent to said imaginarycylinder, the spring-like qualities of the rib means serving to deploysaid sheet into extended position.

8. Apparatus as set forth in claim 7, including control means attachedto the hub and including string means extending to the rib means, andsaid control means being operative to control the rate of deployment ofsheet is non-planar when in extended position.

1. Apparatus for storing on a hub an annular sheet in folded condition and for deploying it into extended position, comprising: a. a hub of diameter which is small as compared with the diameter of the sheet when extended and the hub having an axis; b. a sheet made of thin flexible material fixed in the vicinity of its center to the hub such that when the sheet is in extended position it lies substantially normal to said axis of the hub; c. the sheet having multiple predetermined sharply-defined bend lines about which the sheet is pleated by folding the sheet in one axial direction at multiple major bend lines uniformly spaced about the sheet and then folding the sheet oppositely at intermediate bend lines bisecting the angle between adjacent major bend lines, the folds being made in such a way that at each bend line only the sheet material contiguous thereto is folded upon itself, said major bend lines each extending from the hub as a substantially straight line which is tangent to an imaginary coaxial cylinder located within the hub and of slightly smaller diameter and selected such that extensions of adjacent major bend lines would intersect at common points lying near the surface of the hub; and d. the pleated sheet having sufficient memory of said sharply defined bend lines to assume said folded condition when rolled onto said hub.
 2. Apparatus as set forth in claim 1, wherein the sheet is further pleated to form said segments by folding it in alternating axial directions about minor bend lines which are also substantially straight lines so located on the sheet when in extended position that each minor bend line is parallel to an adjacent major bend line and that the spacings between each minor bend line and an adjacent parallel major or minor bend line is substantially a constant distance measured anywhere on the sheet.
 3. Apparatus as set forth in claim 1, wherein padding spacer means are placed between the pleated segments of the sheet when folded for storage.
 4. In apparatus as set forth in claim 1, including releasable retaining means surrounding the pleated and rolled sheet when in folded condition.
 5. In apparatus as set forth in claim 1, said sheet being deployable by a radially outwardly acting force field.
 6. In apparatus as set forth in claim 1, flexible tube means comprising stiffening rib means fixed to the sheet along each major bend line and each rib means being attached to the hub where the bend line intersects it, the tube means being inflatable to stiffen them and deploy the sheet into extended position.
 7. Apparatus as set forth in claim 1, including spring-like stiffening rib means fixed to the sheet along each major bend line and each rib means being attached to the hub in a position such that when the sheet is in extended position, the rib means lies tangent to said imaginary cylinder, the spring-like qualities of the rib means serving to deploy said sheet into extended position.
 8. Apparatus as set forth in claim 7, including control means attached to the hub and including string means extending to the rib means, and said control means being operative to control the rate of deployment of said sheet.
 9. Apparatus as set forth in claim 7, said control means including motor driven reel means at said hub, string means attached to the reel means and to the rib means, and said sheet being deployed toward extended position when the reel means is driven to unwind the strings therefrom and being wrapped on the hub toward folded condition when the reel means is driven to wind the strings onto the reel.
 10. Apparatus as set forth in claim 1, wherein said sheet is non-planar when in extended position. 